1. Field of the Invention
The present invention relates to a radio communication apparatus and method, and more particularly, to a multi-input multi-output (MIMO) radio communication apparatus and method.
2. Description of the Related Art
One method of increasing a communication capacity without increasing a frequency band, is through the use of multi-input multi-output (MIMO) technology in which a plurality of transmitter antennas are installed. In a space division multiplexing (SDM) method, which is one of the MIMO methods, a data stream to be transmitted is divided into a plurality of sub streams and the sub streams are independently transmitted from respective antennas. After the signals transmitted in parallel through these multi paths are received by a plurality of antennas, the signal is restored to the original data stream. In this way, for example, the quantity of data that can be transmitted in a channel in a wireless local area network (WLAN) can be increased.
In general, in an MIMO method model, the relations among a transmission vector signal (x), a received vector signal (y), and a channel matrix (H) can be expressed as y=Hx. In this case, a channel capacity can be obtained from a transmission power, a channel matrix (H), and the number of transmission antennas.
A technology related to the directional control of each antenna in a radio communication method using an MIMO technology is disclosed in Japanese Patent Application Laid-Open No. 2005-45351 (hereinafter referred to as Patent Document 1) titled “A MIMO-Type Radio Communication System and Radio Communication Apparatus”.
According to the technology, eigenvalues of a channel matrix (H) are calculated, and an estimated value of a channel matrix (H) with respect to eigenvalues which each have a value greater than the average value of the eigenvalues is calculated back. An antenna pattern implementing this estimated value of the channel matrix (H), i.e., an adaptive antenna array, is obtained, thus obtaining desired antenna direction.
However, this technology requires a large amount of computation until an antenna pattern is determined. Also, since the direction of an antenna is controlled so that a current channel matrix can approach to the calculated-back channel matrix, the antenna pattern should be continuous and variable.